1. Field of the Invention
The present invention relates to a lithium secondary battery using an ionic liquid. In more detail, the present invention relates to a lithium secondary battery using an ionic liquid having a bis(fluorosulfonyl)imide anion.
2. Brief Description of the Background Art
Lithium secondary batteries are a small-sized, light-weight rechargeable battery; have a large storage capacity per unit volume or unit weight; and are widely utilized for a mobile phone, a laptop personal computer, a personal digital assistant (PDA), a video camera, a digital camera and the like, and they become absolutely indispensable for various small-sized, light-weight portable devices with relatively large powder consumption. Also, in recent years, middle-sized or large-sized lithium batteries to be mounted in electrically power assisted bicycles or electric vehicles have been developed, and the development is hoped as a measure for making them environmentally friendly.
Polar aprotic organic solvents which are easy to dissolve a lithium salt therein and are hardly electrolyzed have been used as a non-aqueous solvent which is used for a non-aqueous electrolyte of lithium secondary batteries. However, since those organic solvents have a very low flash point, they involve a serious problem regarding safety of a battery such as ignition and explosion to be caused due to generation of heat at the time of overcharge or at the time of short circuit. In particular, in recent years, following downsizing or weight reduction of electronic devices or development of electric vehicles, development of lithium secondary batteries with large capacity and high output is a pressing need, and security of safety becomes an important problem to be solved more and more. For that reason, various investigations are made regarding the use of an ionic liquid as a flame-retardant compound for a non-aqueous electrolyte of lithium secondary batteries.
Above all, an ionic liquid containing a bis(fluorosulfonyl)imide anion (FSI anion) as an anion component has a low viscosity in comparison with other ionic liquids, and the use of the ionic liquid makes it possible to provide a lithium battery having high performance at the time of high-rate charging and discharging, high energy density and high voltage and having high safety due to use of a nonflammable ionic liquid for a solvent of non-aqueous electrolytes (see Patent Document 1).
However, in comparison with batteries using an organic solvent, lithium batteries using a combination of a usual graphitized carbon electrode and an ionic liquid have high internal resistance, poor output performance and low cycle life performance which is caused due to deposition of lithium and decomposition products on the negative electrode surface, and it is the present situation that the lithium batteries do not reach a practical useful level in performance.
For that reason, it is demanded for the liquid secondary batteries using a flame-retardant ionic liquid to be enhanced in battery performance through development of new battery constituent materials or investigations on additives.